Ah, well, I still don't want to discourage you from digging into the Firefox CSS customizations. It is a really useful tool to know.

But for this purpose, you probably want the about:config value browser.uiCustomization.state. It contains all information about UI placement customizations, which you probably want to copy-paste as a whole anyways.

You can create a user.js file in the profile directory to set the about:config value.